In the UMTS (Universal Mobile Telecommunications System) network, the specifications of long-term evolution (LTE) have been drafted for the purpose of further increasing high speed data rates, providing lower delays and so on (see non-patent literature 1). Also, the specifications of LTE-A (also referred to as LTE-advanced, LTE Rel. 10, 11 or 12, etc.) have been drafted for further broadbandization and increased speed beyond LTE (also referred to as LTE Rel. 8 or 9), and successor systems of LTE (also referred to as, for example, FRA (Future Radio Access), 5G (5th generation mobile communication system), LTE Rel. 13 and so on) are under study.
The specifications of Rel. 8 to 12 LTE have been drafted assuming exclusive operation in frequency bands that are licensed to operators (also referred to as “licensed bands”). As licensed bands, for example, 800 MHz, 1.7 GHz and 2 GHz are used.
In recent years, user traffic has been increasing steeply, following the spread of high-performance user terminals (UE: User Equipment) such as smart-phones and tablets. Although more frequency bands need to be added to accommodate this increasing user traffic, licensed bands have limited spectra (licensed spectra).
Consequently, a study is in progress with Rel. 13 LTE to enhance the frequencies of LTE systems by using bands of unlicensed spectra (also referred to as “unlicensed bands”) that are available for use apart from licensed bands (see non-patent literature 2). For example, the 2.4 GHz band and the 5 GHz band, where Wi-Fi (registered trademark) and Bluetooth (registered trademark) can be used, are under study for use as unlicensed bands.
To be more specific, with Rel. 13 LTE, a study is in progress to execute carrier aggregation (CA) between licensed bands and unlicensed bands. Communication that is carried out by using unlicensed bands with licensed bands like this is referred to as “LAA” (License-Assisted Access). Note that, in the future, dual connectivity (DC) between licensed bands and unlicensed bands and stand-alone (SA) of unlicensed bands may become the subject of study under LAA.